EUV lithography is anticipated to be the lithographic process of choice for producing future generations of semiconductor devices having line widths on the order of 27 nm and smaller. The wavelength of the EUV radiation is nominally 13.5 nm, which calls for the use of specialized optics to collect and image the EUV radiation.
One type of EUV optical system used to collect the radiation from the light source is a grazing incidence collector (GIC). A GIC typically comprises one or more concentrically arranged GIC mirror shells configured to receive radiation from the EUV source at grazing incidence and reflect the received radiation in order to concentrate it at an intermediate focus such that the EUV radiation distribution in the far field is uniform to within a specification set by the overall system optical design.
The radiation sources being considered for EUV lithography include a discharge-produced plasma (DPP) and laser-produced plasma (LPP). The conversion efficiency of these sources is only a few percent so that most of the energy used to generate the EUV radiation is converted to infrared, visible, UV radiation and energetic particles that can be incident upon the one or more GIC mirror shells. This broadband radiation causes a substantial thermal load on the one or more GIC mirror shells.
Consequently, each GIC mirror shell therefore needs to be cooled so that the heat absorbed by the mirror does not substantially adversely affect GIC performance or damage the GIC. In particular, the cooling needs to be carried out under high power loading conditions while preventing distortion of the one or more GIC mirror shells. This is because the uniformity and stability of the illumination of the reflective reticle is a key aspect of quality control in EUV lithography. In particular, the intensity and angular distributions of the EUV radiation delivered by the GIC to the input aperture of the illuminator must not change significantly as the thermal load on the GIC is cycled. This requires a specified degree of radiation uniformity in the far field of the GIC radiation pattern, and this uniformity can be compromised by distortion or figure errors in the GIC mirror shells.
To date, essentially all GICs for EUV lithography have been used in the laboratory or for experimental “alpha” systems under very controlled conditions. As such, there has been little effort directed to GIC thermal management systems for GICs use in a commercially viable EUV lithography system. In fact, the increasing demand for higher EUV power in such commercial systems also increases the thermal load on the GIC. Consequently, more efficient and effective thermal management systems must be implemented for GICs for use in commercial EUV lithography systems to minimize the optical distortion due to the thermal load.